Top drive drilling systems have proven to be very reliable and are used extensively in offshore drilling applications.
Use of an overhead top drive eliminates the need for the conventional rotary table and increases drilling efficiencies from 15 to 25 percent over kelly drilling.
Although top drive drilling systems have proven to be very effective, efficient, and safe tools, few are used onshore. Top drive units developed for offshore drilling applications are not readily adaptable for use in onshore applications. The systems require the use of a guide track attached to a mast, which acts as a torque-restraining structure, extending parallel to the direction of the movement of the traveling block for the purpose of preventing rotation of the traveling block and top drive relative to the mast of the drilling rig.
Top drive drilling systems for onshore portable drilling rigs have not enjoyed commercial success in heavy drilling operations for drilling deep wells. Instead, portable power swivel units have been used primarily for drilling wells deeper, working over a well, and for well servicing. These units require a vertically-tensioned cable extending from the drill floor to a suitable point on the mast structure to react the drilling torque from the power swivel. The application of the lateral torque reaction to the vertically-tensioned cable results in significantly increased cable tension, thereby reducing the mast's hoisting capacity. For the most part, portable drilling rigs have been designed to minimize the weight of the mast and associated equipment. Since the guide tracks and torque lines for top drive drilling systems have had to be vertical, the primary advantages of top drive systems have not been heretofore adapted to a mast of a portable land rig whose longitudinal axis is not necessarily parallel to the vertical axis of the well bore.
The invention described herein discloses an improved structure for guiding power swivels used on vertical or inclined, leaning masts which are generally required for heavier drilling operations. The top drive drilling system described is completely portable and usable for onshore applications.
Prior art has heretofore failed to solve the problems in onshore use of top drive drilling systems.
For instance, U.S. Pat. No. 4,667,752 discloses a carriage and vertical guide tracks for preventing rotation of the drive unit about a vertical axis. However, it and other such systems in the prior art require that the guide tracks always be vertical and parallel to the vertical axis to maintain the drive unit above the well opening.